1. Field of the Invention
The present invention relates in general to a control apparatus for controlling a stepped automatic transmission of a vehicle. More particularly, the invention is concerned with a technique for restraining a torque vibration generated immediately after a shifting action of the transmission which is effected while an input shaft of the transmission and an engine of the vehicle are being mechanically connected to each other.
2. Discussion of Related Art
There is known a vehicle equipped with a stepped automatic transmission having an input shaft that is to be mechanically connected to an engine of the vehicle during a shifting action in which the transmission is shifted to a selected one of a plurality of drive positions (gear positions), as disclosed in U.S. patent application Publication US 2003/0127262A1, JP-H09-291838A, JP-H09-308008A, JP-H09-331603A and JP-H10-24745A. For example, in the vehicle disclosed in US 2003/0127262A1, the engine and the stepped automatic transmission are operatively connected to each other through a direct connection clutch which is disposed therebetween in place of a fluid-operated power transmitting device (such as a fluid-operated torque converter) having a high capacity of absorbing a torque vibration. The vehicle disclosed in US 2003/0127262A1 is further equipped with an electric motor provided on each of a rotary input member and a rotary output member of the clutch, so that the vehicle can be driven by the motor while the clutch is placed in its released state, and so that the vehicle can be driven by the engine or by both the engine and the motor while the clutch is placed in its engaged state.
In the vehicle disclosed in US 2003/0127262A1 in which the input shaft of the stepped automatic transmission and the engine are mechanically connected to each other, the speed ratio of the transmission is changed in a step-like manner through the shifting action which is completed for a relatively short length of time. In change of the speed ratio of the transmission, a rotational speed of the engine is rapidly changed to a synchronous speed corresponding to the changed speed ratio of the transmission. When the rotational speed of the engine becomes equalized to the synchronous speed, the change of the rotational speed of the engine is quickly stopped. In this instance, the quick stop of the change of the engine speed causes an elastic torsional vibration in a power transmitting path between the engine and a wheel of the vehicle, thereby leading to a possible occurrence of a shift shock. Such a problem is encountered also in a vehicle in which a fluid-operated power transmitting device equipped with a lock-up clutch is disposed between the engine and the transmission. That is, the vehicle having the fluid-operated power transmitting device suffers from the same problem, when the shifting action of the transmission is effected with the lock-up clutch being placed in its engaged state.